Grain mass flow sensors have become widely used on grain combines to measure and record instantaneous crop yield values, and to compute load, field and farm totals, as grain is harvested. An example of such a system is shown in U.S. Pat. No. 5,343,761, herein incorporated by reference in its entirety.
Although the accuracy of such systems has been good enough allow combine yield monitors to become widely used, farmers and their advisers who analyze their yield data request better accuracy and less effort required to calibrate such systems during field operation. It is known that variable properties of the grain being harvested, such as grain moisture, grain density, kernel size, kernel shape or kernel frictional characteristics can affect the accuracy of grain mass flow measurement, and thus grain yield measurement. The system shown in U.S. Pat. No. 5,343,761 describes the use of two calibration curves (FIG. 6B), one at each of two different grain moisture levels, to allow accurately measuring grain mass flow rate at different moisture levels. Since current yield monitor systems include a real-time grain moisture sensor, it is possible to use the instantaneously measured grain moisture value with the instantaneously measured force on the impact plate, to calculate an accurate instantaneous mass flow rate, by interpolating between, or extrapolating outside, the two calibration curves, each of which is for a specific grain moisture level.
In actual practice, a simpler method of compensating the mass flow sensor's calibration characteristic for grain moisture has been implemented where the curve is either moved or scaled up or down (left or right would also be functionally similar), as in FIG. 6B of U.S. Pat. No. 5,343,761, in relationship to grain moisture. For example, the calibration curve values might be shifted 0.5% for every 1% change in grain moisture from a reference moisture value.
However, there are several problems with the prior art. The first is that dimensional variations in the clean grain elevator housing, chain sprocket and chain paddles from one combine to another, even of the same manufacturer's model, affect the degree to which the calibration curve shifts relative to grain moisture. The second is that the calibration curve does not shift the same amount at all grain flow rates, and the amount that it shifts at different grain flow rates is dependent on an individual combine's elevator component dimensions. The third is that the curve may shift up or down (or alternatively left or right) at different flow rates, and this is also dependent on an individual combine's elevator component dimensions. These problems make it impractical to have highly accurate pre-determined calibration curves, because it is not practical to determine calibration curves for all possible dimensional variations that occur in the components of a combine clean grain elevator.
In addition to these problems it is also known that grain density has an effect on the calibration characteristic of the mass flow sensor. It is further known that grain kernel size or shape can have an effect on the calibration characteristic of the mass flow sensor. It is also known that the frictional characteristics of grain resulting from harvesting conditions, such as air humidity or sticky residue from grain stems; grain maturity, such as grain that is frozen by cold ambient temperatures prior to biological maturity; or differences in grain varieties or hybrids; can have an effect on the calibration characteristic of the grain mass flow sensor.
The method of determining the calibration characteristic of a grain mass flow sensor that is shown in U.S. Pat. No. 5,369,603 (hereby incorporated by reference in its entirety) teaches a method for determining the nonlinear calibration characteristic of a grain mass flow sensor by minimizing the differences between the masses of multiple grain loads, as measured by the grain yield monitor system versus the masses of the same grain loads determined by means external to the combine, such as weighing the loads individually in a grain transport vehicle such as a truck or wagon. This method does a good job of determining the calibration characteristics of the mass flow sensor at different grain flow rates, but does not consider the effect of grain moisture or other grain parameters on the calibration characteristic. Calibration characteristics determined with loads having significantly different moistures or other grain parameters are not accurate for all moistures or grain parameters.
Therefore, despite various attempts at mass flow calibration, problems remain. What is needed is an improved method to compensate for the effects of grain properties on mass flow sensor calibration which may be used in a yield monitor system.